The following photo-catalytic reaction is well-known as a technique to obtain an aimed subject. That is, the light is irradiated to a solid compound which has a photo-catalytic function so as to generate excited electrons or holes. Then a substance is oxidized or reduced by said excited electrons or holes and obtain the aimed subject.
Especially, the photocatalytic decomposition reaction of water is broadly interested from the view point of photo energy convertion. While, a photo-catalyst which shows activity to the photocatalytic decomposition reaction of water can be considered to be a high quality photo functional material possessing functions such as photo absorption, electric charge separation or surface oxidation-reduction reaction.
Kudo, Kato et al are explaining that alkali tantalats are the photocatalyst showing high activity to the stoichiometric photocatalytic decomposing reaction of water by quoting various prior arts [Catal. Lett., 58(1999). 153-155, Chem. Lett., (1999), 1207, Surface Vol. 36, No. 12(1998), 625-645 (shortened to document A)]. In above mentioned document A, there is an explanation about an useful photo-catalytic materials for proceeding the decomposing reaction of water to hydrogen and/or oxygen using a photo-catalyst, and many indications about photo-catalyst used for stoichiometric photocatalytic decomposing reaction of water are mentioned.
Further, they are referring to the photo-catalyst which carrying a promoter such as platinum or NiO.
However, the photo-catalysts explained in these documents are mainly the compound containing oxygen as a non-metallic element. And, since the band gap energy of various solid photo-catalyst is larger than 3 eV, it is difficult to activate it by low energy under 3 eV. The band gap energy can be explained as the width of a forbidden band exists between a valence electron band and a conduction band. On the contrary, almost all of the conventional solid photo-catalyst which can generate electrons or holes by visible-light radiation are unstable under the condition of photocatalytic water decomposing reaction. For example, the band gap energy of CdS or Cu—ZnS are 2.4 eV, but the catalytic reaction is restricted because it is affected by photo-corrosive action, which is corrosive oxidative action.
Almost all of the sun light which reaches to the surface of the earth is the visible light radiation of lower energy. Therefore, for the purpose to progress various photocatalytic reactions effectively, a photo-catalyst which acts by visible light and is the corrosion resistance is necessary. However, among the conventional technique, there is no technique to satisfy the above mentioned requirement.
In the meanwhile, as mentioned above, almost all of the sunlight which can use at the surface of the earth is the visible light radiation and the object of the present invention is to provide a photo-catalyst which can generate an excitation electron or a hole by visible light radiation and is stable under various oxidative and reductive reactions.
Almost all of conventional stable photo-catalysts are containing oxygen as a non-metallic element. In cases of these compounds, the relative energy gap of a valence band and a conduction band is largely controlled by the energy level of oxygen, energy of O 2p orbit, therefore, the band gap energy is small and can not generate photo-catalytic function by visible light radiation. The inventors of the present invention have noticed that a novel photo-catalyst which acts by visible light radiation will be able to be developed if the following compounds with higher valence energy level than O2 is developed. That is, the compound that when an element whose valence electron's energy is higher than that of oxygen is bonded with a metal and hybridize these valence electron orbits, an energy level of the valence energy band is elevated and can reduce the band gap energy and is stable under the photo-catalytic reactive condition.
Since the valence electron of nitrogen atom has higher energy than that of an oxygen atom, the band gap energy of a metal compound containing a nitrogen atom can be made smaller than that of metal oxide. The inventors of the present invention have conjectured that metal and metal compound bonded with suitable amount of nitrogen atoms become possible to generate excitations electron and holes and will become a photocatalyst which acts by visible light irradiation. And have intensive study to find out the compound which is stable under the reacting condition of photocatalytic decomposition of water. Then the inventors of the present invention have found that the compound comprising oxynitride containing at least one transition metal acts as a photo-catalyst and have dissolved above mentioned problem. Many compounds among these compounds form perovskite structure. The stability of said compounds in photo-catalytic reaction is considered to be effected by said crystalline structure.